1. Field of the Invention
The present invention relates to a field-emission cold cathode which emits electrons from sharp tips, a microwave tube using it, and more particularly to such a microwave robe for use in dual-mode pulse operation.
2. Description of Related Art
A microwave robe, such as a travelling-wave robe, a klystron, a gyrotron or the like, may be used in pulse operation turn its output on and off, and further may be used in dual-mode pulse operation which the output of the microwave tube has two values while it is on. Therefore, according to the prior art, a plurality of grids are provided in front of the cathode to control the current quantity emitted from the hot cathode.
As a first example of the prior art, the structure of an electron gun disclose, in Japanese Utility Model Laid-Open No. 60340 of 1989 is shown in FIG. 1. The electron gun 101 has a first grid (shadow grid) 103 and second grid (control grid) 104, both spherically shaped, close to a similarly spherical cathode 102, and an electron beam from the cathode 102, heated by a heater 105, pass through aligned beam transmission holes 110 and 111 of the two grids 103 and 104, respectively, and are electro-statically focused by a focusing electrode 106 and an anode 107. To prevent the electron beam from colliding with the second grid 104, a pulse voltage of hundreds of volts is applied on the second grid 104, and the electron beam synchronized with this pulse voltage is taken out of the cathode 102.
As a second example of the prior art, another electron gun structure disclosed in the U.S. Pat. No. 4,593,130 and the Japanese Patent Laid-Open No. 176851 of 1983 is illustrated in FIG. 2. The electron gun 101 has a first grid 103 and a second grid 104, both spherically shaped, close to a similarly spherical cathode 102, and an electron beam from the cathode 102, heated by a heater 105, and are electro-statically focused by a focusing electrode 106 and an anode 107. This electron gun permits turning on and off in a pulsed manner two modes of current quantity, a high current mode and a low current mode, and the pulse output of a microwave tube using this electron gun can be varied according to the current mode. In a first grid 103, the central part is coarse and the peripheral part is free, while in a second grid 104, the central part is as coarse as that of the first grid 103 and the peripheral part is even coarser. The second grid 104 is always applied with 250 V against the cathode. When in the high current mode, the first grid 103 is biased to +36 V, and electrons are emitted from all over the cathode. When in the low current mode, the first grid 103 is biased to -36 V, and electrons are emitted from only the central part of the cathode, i.e. the coarse part of the first grid 103. At this time, as the potential of the first grid 103 is lower than that in the high current mode, the density of the current emitted From the central part also decreases, and so does the total current. In FIG. 2, a reference lable "+" indicates a plus voltage source and a reference lable ".+-." indicates a plus or minus voltage source.
As a third example of the prior art, still another electron gun structure disclosed in Japanese Patent Gazette No. 52168 of 1991 is illustrated in FIG. 3, and as a fourth example, yet another electron gun structure disclosed in Japanese Utility Model Laid-Open No. 36748 of 1992 is shown in FIG. 4. As seen in FIGS. 3 and 4, the electron gun 101 has a first grid 103, a second grid 104 and from the cathode 102 are electro-statically focused by a focusing electrode 106 and an anode 107. In both the third and fourth examples of the prior art, the electron beam is controlled by three grids, the current being emitted from-all over the cathode when in the high current mode, and from only the central pan of the cathode when in the low current mode.
In the examples of the prior art illustrated in FIGS. 1, 2, 3 and 4, the two grids 103 and 104, or three grids, have to be fixed at high accuracy immediately in front of the cathode 102 whose temperature is as high as 700.degree. C. to 1000.degree. C. Especially in the example shown in FIG. 1, the transmission holes 110 and 111 of the two grids 103 and 104 should be precisely aligned, which means sophisticated labor skills and a long time required for assembly.
In the example of the prior an illustrated in FIG. 1, the voltage of the second grid 104 should be different between the high current mode and the low current mode, but changing this voltage results in a substantial change in the focusing condition of the electron beam, making it impossible to maintain the optimal focusing condition for both the high current and the low current modes. As a consequence, the current ratio between the two modes cannot be raised beyond a certain limit, making it extremely difficult to optimize RF characteristics, such as the efficiency of conversion between DC power and RF power, in both modes.
In the example of the prior art shown in FIG. 2, while the focusing condition of the electron beam does not substantially vary with a change in voltage because, between the two modes, the emitting current instead of the voltage of the first grid 103 is changed but not the voltage of the second grid 104, but the focusing condition is still greatly varied by a change in emitting cathode area. Since the average diameter of the electron beam in the region where the electron beam interacts with RF signals is substantially proportional to the diameter of the cathode, the average diameter of the electron beam is smaller in the low current mode than in the high current mode, with the result that the focusing condition of the electron beam differs between the two modes, and so does, substantially, the amplification gain. In the low current mode, as the diameter of the electron beam decreases to give some clearance between this diameter and the bore of a spiral delay circuit, the current ratio between the two modes can be increased by optimizing the electron beam focusing magnetic field for the current in the high current mode and allowing some ripples for the electron beam in the low current mode, but it still is impossible to optimize the operating condition for both modes.
Moreover, whereas it is necessary to change the first or second grid voltage in order to vary the current ratio between the two modes, this might change the focusing condition and, accordingly, the electron beam transmitting characteristic and the like.